Material-handling equipment, generally referred to as a "tractor loader" or "front-end loader," is one in which the bucket is disposed across the front end. The bucket is usually loaded by moving the bucket into a pile of material after which lift arms are raised and the tractor driven to a new location. At this point, the bucket is swung about the front end of lift arms into a dumping position to discharge the contents, after which the tractor is driven back for another load.
For the most part, loaders are essentially manually controlled devices, which is to say that the manipulation of the bucket through its various positions depends entirely upon the control of the operator. Thus, the operator must move and maintain the bucket level during the movement thereof between the loading and the dumping position. In addition, the operator has to see that the bucket is returned to the digging position before the tractor can be moved to receive another load. Simple as the task may be, this re-positioning of the bucket between each load places a burden on the operator, substantially reducing the speed at which the operator and the machine could perform and, thus, the amount of material that can be moved over any given period of time. Furthermore, the task is fatiguing and unnecessarily requires the operator to devote this attention to what is otherwise a repetitive or routine evolution not requiring special skill.
The tendency for a bucket to spill its contents when manipulated from a lowered to a raised position is due to the characteristic arcuate movement of the lift arms and the bucket. During the raising movement, the bucket is tilted more and more backwardly towards the tractor wherein the material is spilled over the rear edge. This spillage of material is a potential hazard to the operator. In addition, it unnecessarily reduces the effective amount of material moved with each load. Thus, it can be appreciated that any apparatus which would automatically level the bucket during lifting operation, would free the operator to concentrate on driving the tractor and moving the payload at the maximum speed while filling the bucket to its greatest loading capacity.
While various devices have been proposed to maintain the bucket level, many of the devices known to those skilled in the art are extremely complicated in nature and are expensive to incorporate or backfit into the design of the loader. One typical linkage mechanism is shown in the tractor loader invented by Steinkampf (U.S. Pat. No. 3,447,708). Others have incorporated automatic hydraulic circuits to achieve the self-leveling function; Hough (U.S. Pat. No. 2,782,946) is an example of this latter group.